Coated articles are known in the art for use in window application such as insulating glass (IG) window units, vehicle windows, and/or the like. Example non-limiting low-emissivity (low-E) coatings are illustrated and/or described in U.S. Pat. Nos. 6,723,211; 6,576,349; 6,447,891; 6,461,731; 3,682,528; 5,514,476; 5,425,861; and 2003/0150711, the disclosures of which are all hereby incorporated herein by reference.
In certain situations, designers of coated articles with low-E coatings often strive for a combination of high visible transmission, substantially neutral color, low emissivity (or emittance), low sheet resistance (Rs), and good durability. High visible transmission for example may permit coated articles to be more desirable in applications such as vehicle windshields or the like, whereas low-emissivity (low-E) and low sheet resistance characteristics permit such coated articles to block significant amounts of IR radiation so as to reduce for example undesirable heating of vehicle or building interiors. It is often difficult to obtain high visible transmission and adequate solar control properties, combined with good durability because materials used to improve durability often cause undesirable drops in visible transmission and/or undesirable color shifts of the product upon heat treatment.
It will be appreciated that it is desirable for such coated articles to be as durable as possible with respect to mechanical durability and/or chemical durability. In certain example instances, solar control coatings are provided with an overcoat layer of silicon nitride (e.g., doped with aluminum or the like in many instances). The silicon nitride overcoat, however, is subject to durability problems especially when sputter-deposited at high deposition rates.
FIG. 1 of the instant application, for example, illustrates that sputter-deposited silicon nitride layers realize an increase in voids defined therein as sputter deposition rate increases (the data in FIG. 1 is from silicon nitride deposited only via sputtering in a known manner). A large number of voids in an overcoat layer can be undesirable since such voids tend to cause the layer to be porous in nature and be susceptible to attack by environmental elements such as oxygen, water, humidity, cement, and/or the like. When voids are attacked by one or more of such environmental elements, this can be problematic with respect to coating durability and/or coloration. For example, such attacks can result in significant changes in coated article coloration due to oxidation for example, and/or mechanical/chemical damage to the coated article. As another example, significant numbers of such voids may render the coated article subject to unpredictable changes in optical properties (e.g., n and/or k) of the coated article upon heat treatment (HT) in certain instances.
One approach to improve durability of coated articles with solar control coatings has been to apply a diamond-like carbon (DLC) coating to an exterior surface thereof. For example, see U.S. Pat. Nos. 6,461,731 and 6,447,891, the disclosures of which are hereby incorporated herein by reference. While the coated articles of the '731 and '891 patents realize excellent durability, the use of significant amounts of DLC in a coated article is sometimes subject to criticism since it can adversely affect optical characteristics thereof. It has been found that layers such as silicon nitride when deposited via sputtering often end up having dangling bonds therein due to the low energy used in sputter-deposition processing. It would be desirable if a technique could be provided, that is capable of reducing the number of Si dangling bonds in a silicon nitride inclusive layer at least in areas proximate an outer surface thereof.
U.S. Pat. No. 5,569,362 discloses a technique for ion beam treating a coating using at least oxygen in order to densify the same. However, the '362 patent is unrelated to nitrogen doping of Si3N4, fails to disclose or suggest nitrogen grading of a layer, and is undesirable in that it's use of significant amounts of oxygen in the ion beam renders treated layers susceptible to chemical durability problems.
In view of the above, it will be apparent to those skilled in the art that there exists a need for a method of making a coated article having at least one of: (a) improved durability without adversely affecting optical characteristics; (b) reduced dangling Si bonds at least proximate an outer surface of a layer comprising silicon nitride; (c) suitable stress characteristics; (d) N-doping of Si3N4 in at least a portion of a layer in order to improve chemical and/or mechanical durability; and/or (e) less susceptibility to coloration variations upon environmental exposure and/or heat treatment. There also exists a need for corresponding coated articles.